1. Field of the Invention
The present invention relates to a magneto-resistance effect element, a magneto-resistance effect head, a magnetic storage and a magnetic memory that flow sense current in the direction perpendicular to a film surface of a magneto-resistance effect film to detect magnetism.
2. Description of the Related Art
With the discovery of Giant Magnet-Resistance Effect (GMR), magnetic devices, among others, magnetic heads are showing dramatic improvement in performance. In particular, the application of a spin-valve (SV) film to the magnetic head, an MRAM (Magnetic Random Access Memory) and the like has brought about substantial technological progress in the field of the magnetic devices.
The “spin-valve film” means a stacked film composed of two ferromagnetic layers and a nonmagnetic spacer layer sandwiched therebetween, in which the magnetization of the ferromagnetic layer on one side (generally referred to as a “pin layer” or a “fixed magnetization layer”) is fixed by an antiferromagnetic layer or the like to make the magnetization of the ferromagnetic layer on the other side (generally referred to as a “free layer” or a “free magnetization layer”) turn in accordance with an external magnetic field. In the spin-valve film, a great magneto-resistance change can be obtained by the change in the relative angle between the magnetization directions of the pin layer and the free layer.
Conventionally, the spin-valves have been CIP (Current In Plane)—GMR elements passing current in parallel with the film surface. In recent years, a CPP (Current Perpendicular to Plane)—GMR element (hereinafter referred to as the “CPP element”) is receiving attention in that the CPP element can exhibit larger GMR effect than the CIP-GMR element. When considering the application of these magneto-resistance effect elements to the magnetic heads, there arises a problem in view of shot noise and high-frequency response when the resistances of the elements increase. As for the resistance of the element, it is adequate to evaluate it with RA (resistance×current passing area). Specifically, when recording density is beyond 300 Gbpsi (Gigabit per square inch), RA is required to be several hundreds mΩμm to 1 mΩμm2; and when the recording density is 600 Gbpsi, RA is required to be 500 mΩμm2 or below.
With respect to these requirements, the CPP element has a potential to obtain a larger MR (Magneto-Resistance) rate of change even when the resistance is low in the trend of the magnetic devices being miniaturized increasingly. Under such a circumstance, the CPP element and the magnetic head using the same are considered to be potential candidates to realize the recording density of 300 Gbpsi to 1 Tbpsi (Terabit per square inch).
However, in the case of a metal CPP element of which pin layer/spacer layer/free layer (this three-layer structure is called a spin-dependent scattering unit or the spin-valve film) are formed by metal layers, it is difficult to detect a weak magnetic field accompanied by increasing density.
In order to bring a solution to this problem, there is proposed a CPP element using, as a spacer layer, an oxide layer [NOL (nano-oxide layer)] including current pass in the thickness direction (for example, refer to Patent Document 1 (Japanese Patent Laid-Open Application No. 2002-208744)). Such a CPP element can increase both the element resistance and an MR ratio backed by a current-confined-path (CCP) effect. Hereinafter, such an element is referred to as the CCP-CPP element.
Here, along with the trend of microfabrication in the elements (magnetic devices), the size in the plane direction of the CPP spin-valve film is required to be smaller than 100 nm×100 nm. For instance, due to increasing density of the magnetic disk, a track width of 100 nm or below is required.
There is reported that noise caused by a spin-transfer torque arises when letting sense current to flow in the perpendicular direction of the film surface of such a CPP spin-valve film of smaller area. (refer to Generation of STI (Spin-Transfer Induced) Noise, Non-patent Document 1 (M. Covington et al., Phys. Rev. B69, 184406 (2004)); and Non-patent Document 2, (M. Covington et al., J. Magn. Magn. Mater. 287, 325 (2005))).
A spin-transfer torque effect is a phenomenon in which the magnetization direction of the free layer changes, even in a state where no external magnetic field is applied, when passing current of a critical value or more through the CPP spin-valve film. At this time, when the current-passing direction changes, the magnetization direction of the free layer may invert. In other words, when the current-passing direction is fixed to a single direction, the stabilized direction of the magnetization of the free layer is fixed to a single direction. Note that the stabilized direction of the magnetization at this time is described below. When electrons flow from the pin layer to the free layer (current flows from the free layer to the pin layer), a spin transfer torque such that the magnetization direction of the free layer goes along the direction of the pin layer works. Meanwhile, when electrons flow from the free layer to the pin layer (current flows from the pin layer to the free layer), a spin transfer torque such that the magnetization direction of the free layer goes in antiparallel with the magnetization direction of the pin layer works.
In the magnetic head, the magnetization direction of the free layer changes depending on the direction of a medium magnetic field. Therefore, when the current-passing direction is constant, the free layer may become unstable in view of magnetization to induce the noise, depending on the magnetization direction of the free layer. In this manner, even in the state where the sense current is made to flow without applying the external magnetic field, the magnetization in the free layer may become unstable to increase the noise of the element. This noise is called STI (Spin-Transfer Induced) noise since the noise is induced by the spin-transfer torque effect. When the STI noise arises, the noise increases even when a signal output is constant, causing an S/N ratio to degrade to finally increase a BER (Bit Error Rate) in a HDD. As a result, it becomes difficult for the element to detect the weak medium magnetic field at the time of a high density recording, and therefore there is a risk that the element might become substantially unable to be used in the magnetic head and the like.